Polonium-210 Poisoning

Polonium 210 (Po-210) has been identified as the likely source of 'radiation poison' which may have lead to the death of Alexander Litvinenko. The BBC is currently running a story on this event.

Ionactive Consulting Comment

Polonium 210 (Po-210) is an alpha emitter (a type of ionising radiation) with a half life of about 140 days (meaning its activity will reduce by half every 140 days). In simple terms the 'activity' (given the units of Bq or Becquerel) is a measure of how radioactive a substance is - the more Bq the more radioactive. It is important to note that the Bq is not like a measure of mass (e.g. g, Kg). Therefore when one is asked 'how much radiation was Alexander Litvinenko given' it is not simply a question of the physical quantity.

Specific Activity

However, what we do know is that Po-210 has a very high specific activity - this means the amount of activity per unit mass (or Bq/g). This means that a fraction of a micro-g of pure Po-210 contains an incredibly large activity. We can not speculate how Po-210 may have been delivered to the victim, but what is clear is that the physical mass of the material needed would be small - essentially unnoticeable if put in a drink or placed in food. [There is about 166500 GBq or 166500000000000Bq in a 1 g of pure Po-210].

Where is Po-210 found?

Po-210 can be found in cigarette tobacco, drinking water and indeed food (at very low concentrations). In fact we all have very low concentrations of Po-210 in our bodies. Indeed, those that are exposed to naturally occurring radon gas (particularly in Cornwall or other granite bearing areas) are exposed to Po-210 - its a 'daughter' produced by the decay of radon gas. Furthermore, Po-210 is also found in industrial applications such as static eliminator and heat sources for power generation in satellites. Indeed, it is a combination of Po-210 being an alpha emitter and having a high specific activity which actually produces enough decay heat to power small electrical generators.

Po-210: an internal hazard not an external hazard

The fact that Po-210 is an alpha emitter means that it is relatively harmless whilst outside the body - indeed it could sit on the skin with little resulting harm. This is common to most alpha emitters (unless they emit other radiations which are more penetrating). In the case of Po-210 it does have some weak gamma rays but the energy of these is so low as not to be of concern. It is for this reason that Po-210 is difficult to detect when it is 'in something' or 'under something' or 'mixed with something'. In all cases once the alpha particles are in a substance its detection becomes difficult - it would not be detected by simply waving a 'radiation counter' over a contaminated area. In order to detect the alpha particles (e.g. in urine) one would probably have to undertake a process called 'alpha / gamma spectrometry' where the urine would be evaporated down to dryness in controlled conditions. This would then allow the alpha particles (and weak gamma rays) to be detected with sensitive instrumentation and the 'signature' of Po-210 would be seen.

Po-210: physical form and entry route is important

The radiation hazard comes from the inhalation or ingestion of Po-210 since this brings the alpha particles into the region of specific cells in the body (where they can deliver their 'radiation dose'). These cells would include those from the lung, blood, spleen, kidney, liver and bone marrow (the exact location would to some extent depend on how the Po-210 was delivered). However, it's not just the 'radioactive nature' of the Po-210 which dictates its effects - it's also the chemical nature and physical form. For example, its degree of solubility (i.e. chemical form) will have a bearing on how long the material might stay in the body and do harm. Its physical form (particle size) might have an influence on the damage it can do if inhaled (since certain sizes would be inhaled deep into the lung where they would transfer to the blood, where as other sizes would be exhaled). This is one of the reasons why the Health Protection Agency is being cautious at this time - there are many factors to be considered.

Po-210: activity & dose

The media have tended to ask the question 'how much radiation was Alexander Litvinenko given?' or 'what was the dose?'. We must be clear about what we mean by 'dose of radiation'. What he was given was a certain 'activity (Bq)' of Po-210 - it is only when this has entered the body and been incorporated into the various organs / tissues of the body that it can deliver the 'critical damaging dose'. What seems to be clear of course is that enough activity was taken into the body to provide a massive dose of radiation to major organs in the body.

Harm: Low doses vs high doses

When talking about ionising radiation many tend to think of it as a 'cancer enhancing agent'. Indeed at low doses (and by this we mean anything from background radiation, radon, medical diagnostic x-rays and similar), we talk about the enhanced / excess life time cancer risk which can be attributed to a certain exposure (dose) of ionising radiation. What is certain is the uncertainty that this dose will actually cause a cancer. All we know is that as the dose increases the risk / probability of the cancer being caused by the ionising radiation also increases. It is something we live with and it's a simple fact of life - driving a car, taking the bus, smoking and drinking beer are all 'risky'.

However it seems that the radiation dose delivered by the Po-210 incorporated into Alexander Litvinenko has not followed the above expected pattern (or stochastic) radiation effect. Instead it looks like the much less common and more serious 'deterministic' radiation effect which manifests itself in almost immediate and obvious clinical symptoms (hair loss, sickness, organ failure and possibly death). It should be pointed out that this effect is very uncommon - even in accident situations involving sources of ionising radiation. It only occurs after massive doses of radiation have been delivered to the body in a short period of time. There is a whole body and organ specific threshold below which the deterministic effect does not occur.

Unfortunately in this case it seems clear that the threshold has been reached.
The basic unit of radiation exposure is the Gray or 'Gy'. If one were to assume that a whole body could be irradiated uniformly by an external source of radiation (e.g. from an x-ray machine), then effects similar to that noted of the victim would be apparent somewhere between 3-5 Gy (which is a massive dose of ionising radiation by any standards). However, the dose calculation for Alexander Litvinenko will be a lot more complicated as we are not dealing with a nice uniform radiation field interacting with the body. Instead we are dealing Po-210 being incorporated into the body and irradiating cells at very close distance. Many have noted that alpha particles are more damaging than other types of ionising radiation (such as x-rays). For stochastic radiation effects (e.g. cancer induction) it is true that alpha particles can produce around 20 times more damage than x-rays for the same given quantity of absorbed dose. For deterministic effects exposure to the alpha particles will produce similar damage to x-rays but the severity will be greater per unit absorbed dose (i.e. like the stochastic effect). However there is not a factor of 20 difference in damage potential (i.e. like there is for stochastic effects), it is more like 2-4 for the large doses expected to have killed the victim.

How much Po-210 was used?

This is very difficult to establish and Ionactive does not have enough information to predict this with any certainty. However what we can do is provide some pointers as to how this might be worked out. Firstly, we start by looking at the stochastic effect (to simply rule it out). For these effects we can look to the literature to provide us with 'dose per unit intake' - that is the 'dose of radiation per Bq of Po-210'. Even this is not that simple because the values used depend on if the Po-210 was inhaled or ingested and also on the chemical nature of the Po-210 (i.e. what it was chemically attached to). For example, we can look at the ingestion coefficient and find that 833000 Bq would be needed to provide an effective whole body dose of 1 Sv (ICRP 72, worse case assumption for a member of the public). [For this paragraph please assume that 1Sv = 1Gy].

Therefore, one might consider that a small activity in the order of 4000000 Bq would be enough (in mass terms about 0.025 micrograms of Po-210). However, this calculation is only related to stochastic effects at low exposures and is not designed to work with large exposures. In other words, whilst the calculation might tell us that the victim would have certainly died (eventually) from a radiation induced cancer, it does not tell us anything about how he might have died from a deterministic radiation exposure. To summarise - the activity / mass calculated in this paragraph would not be enough to produce the deterministic effects seen in Alexander Litvinenko. Dose calculations based on stochastic dose coefficients - especially where alpha particles are involved is not valid.

Ionactive suspects the actual dose calculations will be based on computer models and by going back to first principles of internal radiation dosimetry (the science that looks at exposures produced by radioactive material in the body). In order to calculate the actual dose some assumptions (some perhaps later proved by analysis) will be required - these include the route of entry (e.g. inhalation or ingestion), the chemical form of the Po-210 compound (e.g. oxide, nitrate), the specific activity (Bq/g), the bio kinetics (i.e how and where the Po-210 is incorporated into the body and for how long) and so on.

However we can still have a stab at estimating an intake based on some very basic concepts and assumptions. For this we need to define a few terms.

(1) Unit of absorbed dose: Gray (Gy) [Units of J/Kg]

(2) Units of decay energy: eV (electron volt) where 1 eV = 1.6 E-19 J

(3) Energy of Po-210 Alpha decay: 5.4 MeV

Here are some assumptions:

(i) Assume mass of body: 70Kg

(ii) Assume that Po-210 incorporates into all vital organs of body [likely]

(iii) Assume dose must be at least 10 Gy (to produce known effects in the victim during the 20 known days of exposure)

(iv) Assume that there is substantial retention of the Po-210 in the body over at least 20 days (i.e. worst case)

(v) Assume that the Alpha particle is 4 times as effective as x-rays in producing deterministic effects (at high doses). [This may not be accurate, but as noted above for a deterministic effect at high exposures, the alpha particle is not 20 times more damaging than x-rays and 4 seems a reasonable assumption].

Given 1Gy = 1 J/Kg, to give a 70Kg body at least 10 Gy (this will ensure bone marrow depression and gastrointestinal effects) will need 700 J. Each alpha decay can provide 5.4 MeV of energy which is 8.6 E-13 J. Therefore the total number of alpha particles required to give this energy will be 8 E14. If we assume the dose was delivered over 20 days then the activity required to do this would be around 462 MBq (462000000 Bq). [This is based on the fact that 1Bq of P-210 will produce 1 alpha per second].

However, since we have assumed that the alpha particle will produce around 4 times as much damage as an x-ray, we can say that ¼ of the activity will provide the same level of dose - i.e. 115.5 MBq.

The 115.5 MBq is probably a lower end estimate. If the required dose were to be delivered over 10 days then this would require at least 231 MBq. Alternatively (or in addition), if perhaps half the Po-210 were to be excreted over 20 days then significantly more activity were be required in order to deliver the same dose.

Sticking with our estimate of 115.5 MBq: this would have a mass in the region of 0.7 micrograms. What is therefore clear is that the mass of Po-210 required to kill an individual by the deterministic radiation effect is miniscule (put it this way, if a grain of sugar represented Po-210 then there would be about 900 deterministic doses in the grain). In actual fact the exact mass (or activity) is totally irrelevant / academic. 1g of Po-210 delivered in food would be clearly devastating and totally undetectable by the victim.